1. Field of the Invention
The present invention relates to an interferometer and method of calibrating the interferometer.
2. Description of the Related Art
Methods of analyzing interference fringes obtained by an interferometer, include a phase shifting method. The phase shifting method is a method of shifting phases of interference fringes by means of, for example, displacing a reference surface in the direction of the optical axis to acquire a plurality of interference fringe images, and calculating the shape of a measuring object. This method is currently applied in many interferometers because it can achieve high accuracy.
The phase shifting method, however, requires a long period of time for measurement because it acquires interference fringes while displacing the reference surface. During the acquisition of interference fringes, it is required to stationarily locate the measuring object. Accordingly, the method can be used only under a particular environment from which shakes such as vibrations are excluded.
In contrast, a method of imaging optically-phase-shifted interference fringes at the same time using a plurality of imaging devices is known from JP-A 2-287107 and JP A 2000-329535. This document discloses a phase shifting method of synthesizing a reference light and an object light to provide a synthesized light, which is split into three through beam splitters. The three split lights are passed through three polarizing plates having different directions of polarization, and projected onto three imaging devices. This method can instantaneously take in a plurality of interference fringe images required for processing in the phase shifting method and accordingly achieve high-speed measurements as well as measurements under vibrating states.
An accurate measurement of the measuring object by the method disclosed in JP-A 2-287107 and JP-A 11-136831 is based on the premise that three phase-shifted interference fringe images have equal biases and amplitudes at corresponding points. The split intensity errors at the beam splitters and the installation errors associated with the fast axis and slow axis of the λ/4 plate occur, however, which make it difficult to equalize biases and amplitudes among three interference fringe images, resulting in a reduction in measurement accuracy.
If an error occurs on installation of the polarization axes of three polarizing plates, the amounts of phase shift between three interference fringe images become different from the design values. Therefore, the interference fringes caused at the surface of the measuring object cannot be given the same amount of phase shift as the design value matching the measurement principle, resulting in a reduction in measurement accuracy as well.
An improvement in measurement accuracy requires accurate production of optical parts contained in the interferometer and adjustment of the interferometer through the use of an accurate adjusting mechanism. A variation with time in interferometer enclosure, a variation in geometric size due to temperature fluctuations and the like, and a variation in performance of optical parts may vary bias values, amplitude values, and phases, which are given immediately after production of the interferometer. Therefore, it is preferable that the user can easily give these values, periodically or before measurement, in the environment in which the user uses the interferometer. This is effective to achieve measurements using an interferometer that is fast, accurate, and independent of use environments.